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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" class = "active" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.<
2022-08-04 06:59:33 +00:00
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2023-06-26 17:46:27 +00:00
< li > < em > 凹语言(专为 WebAssembly 设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
< li > < em > KCL 配置语言(Rust): < a href = "https://github.com/kcl-lang/kcl" > https://github.com/kcl-lang/kcl< / a > < / em > < / li >
< / ul >
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< hr >
< h2 id = "43-map" > < a class = "header" href = "#43-map" > 4.3. Map< / a > < / h2 >
< p > 哈希表是一种巧妙并且实用的数据结构。它是一个无序的key/value对的集合， ， < / p >
< p > 在Go语言中， ， ， ， ， ， ， ， ， < / p >
< p > 内置的make函数可以创建一个map： < / p >
< pre > < code class = "language-Go" > ages := make(map[string]int) // mapping from strings to ints
< / code > < / pre >
< p > 我们也可以用map字面值的语法创建map， ： < / p >
< pre > < code class = "language-Go" > ages := map[string]int{
" alice" : 31,
" charlie" : 34,
}
< / code > < / pre >
< p > 这相当于< / p >
< pre > < code class = "language-Go" > ages := make(map[string]int)
ages[" alice" ] = 31
ages[" charlie" ] = 34
< / code > < / pre >
< p > 因此， < code > map[string]int{}< / code > 。< / p >
< p > Map中的元素通过key对应的下标语法访问： < / p >
< pre > < code class = "language-Go" > ages[" alice" ] = 32
fmt.Println(ages[" alice" ]) // " 32"
< / code > < / pre >
< p > 使用内置的delete函数可以删除元素： < / p >
< pre > < code class = "language-Go" > delete(ages, " alice" ) // remove element ages[" alice" ]
< / code > < / pre >
< p > 所有这些操作是安全的， ； ， ， ， " bob" ]失败时将返回0。< / p >
< pre > < code class = "language-Go" > ages[" bob" ] = ages[" bob" ] + 1 // happy birthday!
< / code > < / pre >
< p > 而且< code > x += y< / code > 和< code > x++< / code > 等简短赋值语法也可以用在map上， < / p >
< pre > < code class = "language-Go" > ages[" bob" ] += 1
< / code > < / pre >
< p > 更简单的写法< / p >
< pre > < code class = "language-Go" > ages[" bob" ]++
< / code > < / pre >
< p > 但是map中的元素并不是一个变量， ： < / p >
< pre > < code class = "language-Go" > _ = & ages[" bob" ] // compile error: cannot take address of map element
< / code > < / pre >
< p > 禁止对map元素取址的原因是map可能随着元素数量的增长而重新分配更大的内存空间， < / p >
< p > 要想遍历map中全部的key/value对的话， ， ， < / p >
< pre > < code class = "language-Go" > for name, age := range ages {
fmt.Printf(" %s\t%d\n" , name, age)
}
< / code > < / pre >
< p > Map的迭代顺序是不确定的， ， ， ， ， ， ： < / p >
< pre > < code class = "language-Go" > import " sort"
var names []string
for name := range ages {
names = append(names, name)
}
sort.Strings(names)
for _, name := range names {
fmt.Printf(" %s\t%d\n" , name, ages[name])
}
< / code > < / pre >
< p > 因为我们一开始就知道names的最终大小， ， ： < / p >
< pre > < code class = "language-Go" > names := make([]string, 0, len(ages))
< / code > < / pre >
< p > 在上面的第一个range循环中， ， ， ， ， < / p >
< p > map类型的零值是nil， < / p >
< pre > < code class = "language-Go" > var ages map[string]int
fmt.Println(ages == nil) // " true"
fmt.Println(len(ages) == 0) // " true"
< / code > < / pre >
< p > map上的大部分操作， ， ： < / p >
< pre > < code class = "language-Go" > ages[" carol" ] = 21 // panic: assignment to entry in nil map
< / code > < / pre >
< p > 在向map存数据前必须先创建map。< / p >
< p > 通过key作为索引下标来访问map将产生一个value。如果key在map中是存在的， ； ， ， " bob" ]那样。这个规则很实用， ， ， ， ， ： < / p >
< pre > < code class = "language-Go" > age, ok := ages[" bob" ]
if !ok { /* " bob" is not a key in this map; age == 0. */ }
< / code > < / pre >
< p > 你会经常看到将这两个结合起来使用，像这样：< / p >
< pre > < code class = "language-Go" > if age, ok := ages[" bob" ]; !ok { /* ... */ }
< / code > < / pre >
< p > 在这种场景下， ； ， ， < / p >
< p > 和slice一样， ； ， ： < / p >
< pre > < code class = "language-Go" > func equal(x, y map[string]int) bool {
if len(x) != len(y) {
return false
}
for k, xv := range x {
if yv, ok := y[k]; !ok || yv != xv {
return false
}
}
return true
}
< / code > < / pre >
< p > 从例子中可以看到如何用!ok来区分元素不存在， ， ： < / p >
< pre > < code class = "language-Go" > // True if equal is written incorrectly.
equal(map[string]int{" A" : 0}, map[string]int{" B" : 42})
< / code > < / pre >
< p > Go语言中并没有提供一个set类型， ， ， ， （ ） ， < / p >
< p > < u > < i > gopl.io/ch4/dedup< / i > < / u > < / p >
< pre > < code class = "language-Go" > func main() {
seen := make(map[string]bool) // a set of strings
input := bufio.NewScanner(os.Stdin)
for input.Scan() {
line := input.Text()
if !seen[line] {
seen[line] = true
fmt.Println(line)
}
}
if err := input.Err(); err != nil {
fmt.Fprintf(os.Stderr, " dedup: %v\n" , err)
os.Exit(1)
}
}
< / code > < / pre >
< p > Go程序员将这种忽略value的map当作一个字符串集合， < code > map[string]bool< / code > 类型value都是无关紧要的； < / p >
< p > 有时候我们需要一个map或set的key是slice类型， ， ， ， ， ， ， < / p >
< p > 下面的例子演示了如何使用map来记录提交相同的字符串列表的次数。它使用了fmt.Sprintf函数将字符串列表转换为一个字符串以用于map的key， ： < / p >
< pre > < code class = "language-Go" > var m = make(map[string]int)
func k(list []string) string { return fmt.Sprintf(" %q" , list) }
func Add(list []string) { m[k(list)]++ }
func Count(list []string) int { return m[k(list)] }
< / code > < / pre >
< p > 使用同样的技术可以处理任何不可比较的key类型， ， ， < / p >
< p > 这是map的另一个例子， ， ， < / p >
< p > < u > < i > gopl.io/ch4/charcount< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Charcount computes counts of Unicode characters.
package main
import (
" bufio"
" fmt"
" io"
" os"
" unicode"
" unicode/utf8"
)
func main() {
counts := make(map[rune]int) // counts of Unicode characters
var utflen [utf8.UTFMax + 1]int // count of lengths of UTF-8 encodings
invalid := 0 // count of invalid UTF-8 characters
in := bufio.NewReader(os.Stdin)
for {
r, n, err := in.ReadRune() // returns rune, nbytes, error
if err == io.EOF {
break
}
if err != nil {
fmt.Fprintf(os.Stderr, " charcount: %v\n" , err)
os.Exit(1)
}
if r == unicode.ReplacementChar & & n == 1 {
invalid++
continue
}
counts[r]++
utflen[n]++
}
fmt.Printf(" rune\tcount\n" )
for c, n := range counts {
fmt.Printf(" %q\t%d\n" , c, n)
}
fmt.Print(" \nlen\tcount\n" )
for i, n := range utflen {
if i > 0 {
fmt.Printf(" %d\t%d\n" , i, n)
}
}
if invalid > 0 {
fmt.Printf(" \n%d invalid UTF-8 characters\n" , invalid)
}
}
< / code > < / pre >
< p > ReadRune方法执行UTF-8解码并返回三个值： ， ， ， ， < / p >
< p > charcount程序同时打印不同UTF-8编码长度的字符数目。对此， ； （ ） ， < / p >
< p > 作为一个实验， ， ： < / p >
< p > < img src = "../images/ch4-xx-01.png" alt = "" / > < / p >
< p > 下面是不同UTF-8编码长度的字符的数目： < / p >
< pre > < code > len count
1 765391
2 60
3 70
4 0
< / code > < / pre >
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< p > Map的value类型也可以是一个聚合类型， ， ， ， ， < / p >
2022-08-04 06:59:33 +00:00
< p > < u > < i > gopl.io/ch4/graph< / i > < / u > < / p >
< pre > < code class = "language-Go" > var graph = make(map[string]map[string]bool)
func addEdge(from, to string) {
edges := graph[from]
if edges == nil {
edges = make(map[string]bool)
graph[from] = edges
}
edges[to] = true
}
func hasEdge(from, to string) bool {
return graph[from][to]
}
< / code > < / pre >
< p > 其中addEdge函数惰性初始化map是一个惯用方式， ； ， < / p >
< p > < strong > 练习 4.8： < / strong > 修改charcount程序， ， < / p >
< p > < strong > 练习 4.9： < / strong > 编写一个程序wordfreq程序， < / p >
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